Fuel cells having a membrane electrode assembly for generating an electric power from a fuel and an oxidizer are known. FIG. 11A shows such a related art fuel cell 50. The fuel cell 50 includes a polymer electrolytic membrane (PEM film) 52b having a proton conductivity, a cathode electrode 52a defined on one side of the polymer electrolytic membrane, and an anode electrode 52c defined on the opposite side, to form a membrane electrode assembly (MEA) 52. Electrochemical reaction between oxygen in the air supplied to the cathode electrode 52a and hydrogen in the fuel gas supplied to an anode electrode 52c generates an electric power. The fuel cell 50 is formed by laminating or stacking a plurality of unit cells 51, each generating a unit of electric power.
The fuel cell 50 comprises, as shown in FIG. 11B, an inlet 53a and an outlet 53a1, of hydrogen gas, an inlet 53b and an outlet 53b1, of the air, and an inlet 55b and an outlet 55b1, of cooling water.
FIG. 11C shows a related art cooling system for the fuel cell 50 in which cooling water is circulated. The cooling system comprises a radiator 54 as a heat exchanger, a cooling water passage 55, a pump 56 for circulating the cooling water, and a thermostat valve 57 for circulating the cooling water toward a bypass passage 55a to bypass around the radiator 54 on cooling.
The cooling water circulates through the cooling water passage 55 by the pump 56, enters the fuel cell 50 at the cooling water inlet 55b, passes through the inside of the fuel cell 50, and exits at the cooling water outlet 55b1 into the cooling water passage 55. During this, the cooling water, as shown in FIG. 11A, passes through the grooves 51c formed in separators 51a and 51b. 
To avoid the shunt current through the fuel cell 50, a mixture of ethylene glycol having a high electric insulation and pure water or the like is used as the cooling water. Further, to absorb ions generated in the cooling water due to thermal degradation accompanied with the operation of the fuel cell 50, as shown in FIG. 11C, an ion exchanger 58 is generally provided in the cooling water passage 55.
In this fuel cell 50, the cooling water may leak in a membrane electrode assembly (MEA) 52 in the fuel cell 50 because the cooling water directly circulating through the fuel cell may cause a trouble or a power generation loss.
Further, to avoid the shunt current through the fuel cell 50, the performance of the ion exchange must be maintained, which requires a periodical exchanging the ion exchange resin in the ion exchanger 58. Thus, in the fuel cell 50 shown in FIG. 11A, there is a problem that the fuel cell 50 cannot use an electrical conductive coolant (long life coolant), which has been used in the vehicle using an internal combustion engine.
Further, the cooling water including ethylene glycol or the like has a tendency that the viscosity extremely increases at a low temperature of the fuel cell 50. Thus, there is a problem that the pump 56 receives a high load at a low temperature and thus, the efficiency becomes low because the pump 56 should force the coolant having a high viscosity to flow through the narrow grooves 51c in the separators 51a and 51b. 
On the other hand, in the field of art of the fuel cell, a technique for cooling the fuel cell using a heat pipe is known. In the heat pipe, a working liquid is enclosed as a heat medium (heating medium, heat carrier) and provided between unit cells to cool the fuel cell by natural circulation. Further, a heater is provided separately from the heat pipe to heat the fuel cell (warm-up operation).
Japanese Laid open patent application No. 5-121083 discloses a fuel cell having a heat pipe sandwiched between unit cells within the cell to uniform the heat distribution in the cell.
Japanese Laid open patent application No. 11-214017 discloses a fuel cell having a heat pipe unit laminated between unit cells and extending from the laminated part to be exposed to the external.
However, the fuel cells disclosed in these documents are cooled by natural circulation of the working liquids in the heat pipes, so that an effective cooling or an effective warm-up operation cannot be provided.